Lots of reasons why one might pay more for the offload card. If you're
cheap, you'll just use the software stack and a $10 NIC and just live
with the corresponding CPU usage. If you're a performance freak, you'll
spend whatever you have to to squeeze out every last bit of performance
you can.

Mind you, another option is, if you're dealing with the kind of load
that requires that much network performance, is to use redundant
servers, like google. No one server is exceptionally fast, but it not
many people are using it, it's fast enough.